Oil drilling is a general term for any boring through the Earth's surface that is designed to find and acquire underground assets such as hydrocarbons. A drillstring on a drilling rig is a column, or string, of drill pipe that transmits drilling fluid and torque to the drill bit. Downhole measurement of drillstring attitude is typically performed for the purpose of steering and automation of drilling tools in directional drilling. The measurement is known as surveying and includes the calculation of the inclination and azimuth of the drillstring with respect to the earth's gravity and magnetic fields. The measurement performed when the drillstring is stationary is referred to as a static survey. The measurement performed when the drillstring is drilling is referred to as a continuous survey.
In current technology, the attitude (i.e., the inclination and azimuth, referred to as survey parameter) is computed using 3-axis accelerometer and 3-axis magnetometer sensor measurements (collectively referred to as survey data). Raw data of these measurements along three mutually-orthogonal axes is referred to as 3-axis data. Normalized projection of the 3-axis data on the axial axis is referred to as normalized axial data. Normalized projection is computed by dividing the axial component of the accelerometer/magnetometer data by the total gravity/magnetic field. The “axial” axis is the drilling axis of a downhole tool and generally referred to as z-axis, as is known to those skilled in the art. The calculation may be performed using 3-axis data (referred to as 3-axis measurement method known in the art) or using normalized axial data (referred to as axial vectors method known in the art). The axial vectors method using the normalized axial data is preferred during drilling when the magnitude of the angular velocity of the drillstring is high. In particular, this avoids using the lateral axis accelerometer data, which can be corrupted by centripetal and tangential components during drilling. However, when the axis of the drillstring at the sensors is substantially parallel (e.g., within +/−10 deg) to the earth's gravity and magnetic fields, sensor noise can lead to large errors in the calculations. In other words, the 3-axis measurement method using 3-axis data becomes less accurate during drilling due to shock/vibration induced noise sensitivity of the sensors. The axial vectors method becomes less accurate when the tool is drilling near vertical or drilling in the north-south plane due to gravity/magnetic field induced noise sensitivity of the sensors. Specifically, the axial vectors method using axial data to calculate inclination becomes less accurate when the tool is drilling near vertical. Similarly, the axial vectors method using axial data to calculate azimuth becomes less accurate either when the tool is drilling in the north-south plane.